Practical methylation of aryl halides by Suzuki-Miyaura coupling

Article abstract of DOI:10.1016/S0040-4039(00)01038-8

A number of aryl halides (X = Cl, Br, I) can be converted to the corresponding toluenes in an operationally simple manner using trimethylboroxine (TMB) as a partner for palladium-catalysed Suzuki-Miyaura coupling. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01038-8

Tetrahedron Letters 41 (2000) 6237±6240
Practical methylation of aryl halides by Suzuki±Miyaura
coupling
Matthew Gray,* Ian P. Andrews, David F. Hook,y John Kitteringham
and Martyn Voyle
Synthetic Chemistry, SmithKline Beecham Pharmaceuticals, Third Avenue, Harlow, Essex, CM19 5AW, UK
Received 16 May 2000; accepted 20 June 2000
Abstract
A number of aryl halides (X=Cl, Br, I) can be converted to the corresponding toluenes in an opera-
tionally simple manner using trimethylboroxine (TMB) as a partner for palladium-catalysed Suzuki±
Miyaura coupling. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: trimethylboroxine; methylation; aryl halide; palladium; Suzuki±Miyaura.
In the course of recent studies we became interested in methods for regiospeci®c incorporation
of a methyl group into aromatic moieties. An obvious strategy is a metal-catalysed cross-coupling
reaction of an organometallic methyl species `MeM' and an aryl halide ArX (where M=Sn, Mg,
Zn, B, Al. . .). In particular, the Suzuki±Miyaura coupling (M=B) appeared attractive to us, with
mild conditions, broad functional group tolerance, non-toxic and easily removed by-products and
conveniently handled reagents as notable features.1
The prominent variant of this reaction is sp2±sp2 coupling, although sp3±sp2 coupling is also
well established.2 7 There are limited reported examples using methylboron derivatives; methylboronic
acid (MBA) has been moderately useful to date,4 6 whereas methylboranes derived from 9-BBN
are more reactive but less readily available.2,3,7 We were discouraged from the use of MBA
because it is expensive and not readily available.
We now report that the anhydride, trimethylboroxine (TMB),{ is a useful and cheaper alter-
native reagent for methylation (Scheme 1). It has been used previously as a methylating agent for
nickel catalysed coupling with allylamines.8,9
* Corresponding author. Fax: +1279 622348; e-mail: matthew_gray-1@sbphrd.com
y
{
Current address: Department of Chemistry, University of Cambridge, Lens®eld Road, Cambridge, CB2 1EW, UK.
Trimethylboroxine is commercially available on a large scale from Callery Chemical Co., Pittsburgh, PA 15230,
USA.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01038-8

6238
Scheme 1.
A number of aryl halides were investigated to test the scope of the method (Table 1). Reactions
were conducted at 100±115ꢀC using tetrakis(triphenylphosphine)palladium(0) as catalyst,
inexpensive potassium carbonate as base and one equivalent of the trimeric TMB.
The results shown in Table 1 are largely unoptimised,6 although preliminary optimisation has
been carried out with the model case of 4-bromobenzophenone. The usual broad functional
group tolerance of Suzuki±Miyaura coupling is mirrored in our results.
Methylation of electron-poor aryl bromides is ecient (entries 1±3), whereas with more
electron rich substrates (entry 4), prolonged reaction times are required, although these can be
shortened by the use of the polar solvent DMF at ca.115ꢀC.
Aryl chlorides are usually more readily available (and hence cheaper) than the corresponding
aryl bromides or iodides, and pleasingly electron-poor chlorides, including a heteroaromatic
chloride,6 provide the methylated products in good yield (entries 5±7). With aqueous dioxane as
solvent, TLC analysis indicated complete consumption of starting material within a few hours
(entries 6 and 7). Of note is the dimethylation of a dichloro substrate (entry 7) which indicates
that nitrotoluenes can enter into the cross-coupling.x Signi®cantly, entries 8 and 9 (naphthyl
halides) demonstrate that, under largely unoptimised conditions, substrates without activating
groups present can also be methylated in moderate to good yield.{ Yields obtained with the
cheaper TMB are comparable to those obtained with MBA.
Using 4-bromobenzophenone as substrate, the e€ect of di€erent bases, solvents, catalysts and
methyl transfer reagents was investigated, primarily to address reaction rate. A number of
experiments to generate relative data (cf. control conditions: Table 1, method A) were carried out
using an SK-233TM workstation with on-line HPLC analysis. The inexpensive base potassium
carbonate was one of the better studied, although cesium bases led to the highest conversions (e.g.
95% isolated yield with Cs2CO3). In the absence of base, only low level conversion is achieved.
Aqueous dioxane or toluene were notable improvements over control conditions A (dioxane).
Here isolated yields were essentially quantitative and, with the former, consumption of starting
bromide was rapid. The best catalyst studied was PdCl2(dppf), giving complete conversion within
3 h.** Reactions with TMB in aqueous dioxane and with MBA in dioxane were both complete
within 3 h. Using TMB in aqueous dioxane may involve in situ generation of MBA since
hydration of TMB is a facile process.11 MBA esters MeB(OR)2 were inferior to TMB.
Treatment of the dichloride with Me3Al10 (1.1 equiv. DMF, 10 mol% Pd(PPh3)4, 70ꢀC internal, overnight) leads to
monomethylation ortho to the nitro function (84% yield).
x
{
1
Complete consumption of 5-bromo-m-xylene was achieved under conditions C (24 h) as evidenced by H NMR and
TLC analysis of the crude product; however, the puri®ed yield of mesitylene was low, possibly due to volatility.
**The cheaper and more robust Pd(OAc)2/4PPh3 combination has proved comparable to Pd(PPh3)4 for certain sub-
strates.

6239
Table 1
Methylation of aryl halides with trimethylboroxine (TMB)
In conclusion, trimethylboroxine (TMB) is a practical methylating agent for a variety of aryl halides
under palladium catalysis.
Typical experimental procedure (entry 7)
1,5-Dichloro-2-nitro-4-(tri¯uoromethyl)benzene (1.02 g, 3.92 mmol), potassium carbonate
(1.63 g, 11.76 mmol), Pd(PPh3)4 (0.45 g, 0.39 mmol), 10% aq. 1,4-dioxane (10 mL) and TMB

6240
(0.55 mL, 3.92 mmol) were charged to a ¯ask and the contents heated to 105±115ꢀC (oil bath
temperature) under nitrogen for 6 h and then stirred overnight at ambient temperature. The
reaction mixture was ®ltered through a pad of Celite1, washed with THF and concentrated in
vacuo. Flash column chromatography (SiO2, 10:1 hexane:Et2O) a€orded 1,5-dimethyl-2-nitro-4-
1
(tri¯uoromethyl)benzene as a low melting solid, 0.72 g (84%): Rf 0.45 (5:1 hexane:Et2O); H
NMR (400 MHz, CDCl3) ꢀ 8.27 (s, 1H), 7.30 (s, 1H), 2.65 (s, 3H), 2.54 (q, 5J(H,F)=1.5 Hz, 3H);
13C NMR (100 MHz, CDCl3) ꢀ 146.5 (bs), 142.3 (q, 3J(C,F)=1.5 Hz), 137.5 (q, 5J(C,F)=1.0 Hz),
136.5 (bs), 127.8 (q, 2J(C,F)=31.7 Hz), 123.3 (q, 1J(C,F)=273.6 Hz), 122.7 (q, 3J(C,F)=5.4 Hz),
4
20.3 (s), 19.0 (q, J(C,F)=2.4 Hz); LRMS (EI+) m/z (relative intensity) 219 (M+, 22), 202 (100),
189 (5); HRMS (EI+) m/z calcd for C9H8NO2F3 219.0507, found 219.0496.
Acknowledgements
The invaluable support of Mrs G. Smith and Mr A. Bateman with the SK-233TM workstation
and on-line HPLC analysis is gratefully acknowledged.
References
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